ИСТИНА

Geomorphological evidence in the Altai Mountains, in southern Siberia, indicates that catastrophic drainage of glacier-dammed lakes occurred at some point in the past. The evidence includes giant ripples in the valleys of large rivers, spillways, and large boulders transported considerable distances. These fluvial landforms can only have formed with enormous river discharge associated with outburst floods, and are presumed to have occurred during a period of deglaciation. One of the most notably catastrophic events in Altai is the breakthrough of the glacier-dammed lake that occupied the Kurai Basin. Despite the long history of study, there is still no consensus among researchers about over which period the lake existed, or when the drainage occurred - although recent cosmogenic ages indicate a post-LGM timing. The main problem is the lack of data on the age of the sediments formed by the outburst flood. We have characterized flood relief forms in the valleys of the Chui and Katun rivers; these include terraces, blocky placers, gigantic ripples (channel forms), and glacial moraines that presumably blocked the river flow. The use of standard OSL dating of sandy grains is associated with a number of difficulties – insufficient signal, poor zeroing during transport and considerable mixing of material from various sources (Fig. 1). Because of this, we have attempted date the flood events using rock-surface burial dating. We make use of granite cobbles from within the terrace body, and large boulders emplaced on the surface. Additional samples come from within mega-ripple deposits, and from loess coverings on some of the terrace sections. This research was supported by Russian Science Foundation (project 19-17-00179) Baker V.R., Benito G., Rudoy A.N. Paleohydrology of late Pleistocene Superflooding, Altay Mountains, Siberia // Science, 1993. Vol. 259. Р. 348—352.